A number of closed-cycle thermodynamic machines operate with heat from a high-grade source, typically in the form of a fossil fuel burner, to directly heat a heat exchanger or heater head integral with the machine. A Stirling engine is an example of such an external combustion thermodynamic machine.
Stirling engines are well suited to cogeneration (or combined heat and power) applications and offer significant potential advantages. Stirling engines have a low emission external combustion system, thereby allowing multiple fuels to be used, preventing internal parts becoming fouled by the combustion products and providing quiet operation. However to realize these advantages in systems for the mass market, not only are issues of working lifespan, noise and emissions important but efficiency of fuel use and maintaining a cool engine compartment are key considerations. For maximum efficiency it is necessary to recover a portion of the latent, as well as the sensible heat of the water vapor formed in the combustion process. Moreover, particularly for domestic applications, size, cost, simplicity of construction and maintenance of the systems become very significant considerations.
A burner of the applicant's design, for use in a Stirling engine, is described in WO/9940309. The heating system used therein is one in which the combustion products from the burner are first sent through a heat exchanger or heater head for transferring heat to the working fluid of the Stirling engine. This compact burner provides recuperative heat exchange means in a device that can be economically manufactured. When used in a micro-cogeneration system, the exhaust gases from the burner pass to a separate (preferably condensing) exhaust gas heat exchanger for the production of hot water for space and potable water heating.
However, this type of configuration poses disadvantages in this and other systems utilizing a conventional exhaust gas heat exchanger. The disadvantages largely stem from the incorporation of the heat exchanger as a subsequent addition to the engine rather than as an integral engine design element. In particular, disadvantages in conventional systems include:                The heat exchanger requires connection to the engine exhaust via some form of coupling or duct. In a typical configuration, a rigidly mounted heat exchanger is connected to an engine mounted on vibration isolators and thus a flexible exhaust coupling is required. The complexity and cost of producing a reliable exhaust coupling are increased by the harsh environmental conditions including the effects of corrosive high temperatures exhaust gases.        Heat is lost directly from the coupling. The inherent design constraints of a non-integrated flexible exhaust coupling hinder the efficient prevention of heat losses.        Constrained and/or uneven gas flow.        Increased manufacturing and operational system complexity and thus cost.        Heat is only recovered from the exhaust gases and not from other heated engine surfaces/components.        Separate thermal insulation surrounding the engine is required to prevent heat losses and undesirably high exterior surface temperatures.        Additional heat exchanger fluid connections are necessary.        Increased system dimensions        Increase system weight        
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.
Any publication cited in this specification is hereby incorporated by reference, however this does not constitute an admission that the document forms part of the common general knowledge in the art, in New Zealand or in any other country. The applicant reserves the right to challenge the pertinency of any publication cited herein, or to challenge the accuracy of any assertion made in a cited publication. As used herein, the word “comprises” means “includes, but is not limited to” and its derivatives have a corresponding meaning.